This invention relates to a tubular reactor of the type adapted to receive a pressurized gaseous medium at a temperature within the range of approximately 600.degree. and 1000.degree. C after heating in a nuclear reactor for carrying out an endothermic chemical reaction. More particularly, the present invention relates to such a tubular reactor for cracking hydrocarbons with water vapor to generate gases which contain CO, H.sub.2, CH.sub.4 and CO.sub.2 by utilizing the sensible heat of hot gases, particularly a rare gas, such as helium, under a pressure within the range of 10 to 100 bars after heating as aforesaid in a nuclear reactor to thereby meet the heat requirements to carry out the endothermic reaction.
It is known in the art to conduct a heated gas through an insulated pressure vessel having suspended therein reaction tubes which are usually filled with a catalyst. Such reaction tubes undergo substantial elongation when heated from a cold state up to an operating temperature. The reaction tubes are, therefore, provided in the form of jacketed tubes, each comprising an outer reaction tube and an internally-disposed return tube in which the entry of the gas mixture for the reaction and the exit of the reaction gas are situated on one side. The gas mixture for the reaction flows upwardly in an annular space, usually filled with a catalyst, between the outer reaction tube and the inner return tube. The reaction is completed at the bottom of the annular chamber and the reaction gas is discharged upwardly from the reaction tube through the return tube. The sensible heat delivered by the gas for example, helium, flows around the reaction tube in an upward direction which is countercurrent to the flow of the reaction mixture. By this relation, the top point of the reaction tube is situated in the relatively cool part of the helium flow and, therefore, can be constructed as the fixed point. The tubes are allowed to expend freely in the downward direction, thus facilitating the use of means for passing the pigtails from the reaction tube through the wall of the pressure vessel.
Since there is no substantial radiation properties to the heat-delivering gas, it follows that heat is transmitted to the reaction tubes predominantly by convection. Several proposals have been suggested to achieve optimum heat transfer coefficients at the heating side of the vessel.
Deflection plates, sometimes called baffles, of the type conventionally employed in heat exchangers to achieve a crossflow of helium, have also been used for gas-heated reaction tubes. The use of baffles requires an additional arrangement of support tubes on which the deflection plates are mounted. Moreover, the minimum distance between the reaction tubes is limited by the factors relating to mechanical strength and manufacturing processes which bring about a requirement to retain a sufficiently wide web between the individual bores. These factors and the additional requirement for support tubes have brought about the need to employ very large diameter pressure vessel only at high cost. A further disadvantage of these known constructions resides in the relative motion occurring between the reaction tubes and the baffles as a result of heating and cooling and which can initiate vibrations in the system. It is, therefore, possible for the pressurized reaction tubes to be destroyed at the point of contact between the reaction tubes and the deflection plates.
In another known arrangement wherein the heatdelivering gas flows through an annular gap in an axial direction with respect to the reaction tube, a block of carbon or sintered, high-purity alumina of a length corresponding to the active length of the reaction tubes, in inserted into the interior of the pressure vessel. The block, made of individual layers, is drilled axially with respect to the pressure vessel so that after insertion of the reaction tubes into the drilled openings, an annular gap is formed for the flow of the heatdelivering gas. Since a web must be retained between the bores drilled in the block, a minimum distance of 170 millimeters will be obtained with the materials in use, for example, in the case of a reaction tube with an external diameter of 120 millimeters. A further reduction to the pitch between tubes is, therefore, not possible.
It is also known in the art to employ a support plate for the top ends of the reaction tubes. The support plate includes longitudinal bores with a diameter corresponding to the diameter of the reaction tubes which are inserted into the bores and secured to the plate by welding. The support plate is relatively thick to provide the required mechanical strength and necessary guiding for the reaction tubes. Moreover, the tubes in the bore can only be secured by welding from the top. A reduction to the distance between the tubes is not possible particularly in relation to the last-mentioned construction.